This invention relates to a method for making silicon carbide fibers of high purity and high density, and to the fibers themselves.
Silicon carbide is recognized as having excellent properties for high temperature applications. Silicon carbide articles have been made by sintering silicon carbide powder as taught in U.S. Pat. Nos. 4,144,207, 4,663,105, 4,693,988 and 4,702,869. However, formation of the desired shapes from particles can be difficult, and the use of silicon carbide particles does not lend itself to the manufacture of silicon carbide fibers.
Seyferth and Wiseman, Am. Ceram. Soc., July 1984, pp. C132,133, report combining silicon carbide particles and polysilazane and sintering the mixture in a nitrogen atmosphere to form a bulk ceramic object. Under the conditions used, the polymer yields a substantial amount of silicon nitride. Fibers from the polymer are reported, but not from the combination of polymer and silicon carbide particles. U.K. Pat. No. 2,024,789 describes particular silanes from which shapes can be formed and sintered in an inert atmosphere to make silicon carbide shapes. Green fibers drawn from polymer are mentioned and the use of powdered silicon carbide as a filler is mentioned. However, there is no description of forming fibers from a powder/polymer mixture.
Silicon carbide fibers have been manufactured by drawing silicon-containing organic polymers into fibers and pyrolysing such fibers to produce silicon carbide fibers. This is taught in U.S. Pat. No. 4,126,652, EP No. 0 200 326, and U.S. Pat. No. 4,052,430. In a somewhat similar way, silicon metal can be incorporated in pitch or polyacrylonitrile fibers and pyrolysed to produce silicon carbide fibers. See U.K. No. 1,514,171, U.K. Application No. 2,066,800 and U.S. Pat. No. 4,126,652. However, none of these polymers when spun into fibers are sufficiently stable to be sintered without first partially oxidizing in order to cross-link the polymers. This introduces impurities into the final silicon carbide fibers that are difficult to remove. It prevents reaching high purity levels which in turn prevents achieving densities approaching theoretical and prevents achieving optimum final silicon carbide fiber properties.
U.S. Pat. No. 4,604,367 reports that boron containing polycarbosilanes are spun and irradiated to crosslink the polymers so as to eliminate oxygen. The strengths of such fibers are better at high temperatures than those in the previous examples and the X-ray pattern indicates extremely small grain size of entirely beta silicon carbide. The irradiation takes up to one hour.
Silicon carbide fibers have also been manufactured by chemical vapor deposition of silicon carbide on a substrate such as carbon fiber. Chemical vapor deposition can yield highly pure high density silicon carbide, but this method cannot produce a fiber which is silicon carbide throughout, and is difficult and expensive. Chemical vapor deposition is taught in U.S. Pat. Nos. 4,068,037 and 4,702,960.
This invention provides a method to make dense, high purity silicon carbide fibers which do not require the use of a substrate in their manufacture.